The demand for alternative or renewable energy sources has dramatically increased during the last decade of the 20th century and continues in the new millennium. This demand is driven by the awareness of the depletion of the earth's non-renewable carbon based sources of energy of oil, gas and coal, and by the apparent effects of carbon waste emissions in particular, carbon dioxide and sulfur and nitrate emissions, adding to global warming and pollution of the planet's renewable air and water biosphere. The conversion of wind energy to usable electrical energy is generally considered to be the most promising source of renewable energy for the 21st century. This is evidenced by an increase of nearly seven fold of the power production through wind power from around 2000 megawatts in 1990, to more than 13,400 megawatts at the end of 1999.
Ideally, the conversion of wind energy to usable electrical energy wold be pollution free and have a zero negative impact on the earth's environment. In practice however, the use of popular wind turbines having rotor blades as long as 80 meters or longer and having tip speeds exceeding 100 mph introduce serious environmental hazards including visual and noise pollution and, perhaps of most immediate public concern, bird strikes. Bird mortality is a fact that proponents of modern wind turbines do not like to advertise and it is difficult to obtain bird mortality statistics. Dealers of wind equipment do however caution the use of such equipment within known or potential bird migratory routes, or within locations where threatened or endangered bird species live and nest. Rotor blade driven wind turbines pose a particular hazard to raptors, or birds of prey, many species of which are classified as endangered. The hazard is amplified by the fact that wind farm sites are generally chosen or maintained clear of trees and bushes, and are often populated by small rodents or rabbits, prey of raptors or hunting birds. The observed behavior of these birds is to focus so intently on the object of prey that they do not become aware of the hazard and are often struck and killed by the high speed rotor blades. It is not practical or cost effective to attempt to fence out or otherwise exclude prey of these birds from wind farm sites. It is clear that a more wildlife and habitat friendly wind energy conversion device is demanded. HSB Insurance Company, a major specialty insurer, published a February, 2004 article in “The Locomotive” entitled “The Changing Face of Wind Power”. This article provides a brief summary of safety issues relating to environmental protection and in particular to aviary safety. U.S. Pat. No. 6,623,243 entitled “Minimization of Motion Smear, an Approach to Reducing Avian Collisions with Wind Turbines” to Hodos, (2003) emphasizes the need to address the serious problem of bird strikes by modern high speed wind turbine rotor blades. Hodos' work introduces the use of highly contrasting patterns in an attempt to reduce the ‘motion smear’, or motion blur that causes the rotor blades to appear nearly invisible to birds in flight which approach wind turbines or wind farms having wind turbines of this type. Of the large number of wind turbine designs that have been produced especially over the last 40 years, the shrouded turbine is the only design that has the potential both to save birds harmless and provide the wind energy conversion efficiency and cost efficiency demanded of the application. Shrouded turbines generally allow the use of smaller and more enclosed rotor blades or impellors, and have physical shrouds or ring shaped concentrator wings that are highly visible to birds in flight but at the same time do not present moving objects, such as large rotating blades that are considered by many to visually mar the natural landscape. Of the shrouded wind turbines, versions having two or more concentrator wings that allow the wind to flow between the concentrator wings and develop a vacuum or suction that drives the turbine, have demonstrated, in recent times, to be the most promising and efficient devices. A main object of the present invention is to make improvements to wind energy conversion devices of this type, such that these devices have greater conversion efficiency, and are made more practical and cost efficient.
Several wind energy conversion devices of note employ shrouds or concentrator wings to accelerate the flow of wind through the turbine that converts energy usually to usable electrical energy. U.S. Pat. No. 5,599,172 entitled “Wind Energy Conversion System” to McCabe (1997) provides an example of a single shroud surrounding the turbine rotor blades and includes a description of the shroud acting as a venturi. The venturi is a commonly understood device used to develop a drop in static fluid pressure, air in this case considered to be the fluid, and thereby increase the flow velocity in the restriction or neck of the venturi. U.S. Pat. No. 4,075,500 entitled “Variable Stator, Diffuser Augmented Wind Turbine Electrical Generation System” to Oman, et al. (1978) provides an additional example of a turbine that includes a surrounding shroud having unique perforations that are alleged to “energize the boundary layer along the wall of the diffuser”, acknowledging the importance of maintaining an attached or laminar flow of air through the aft end of the diffuser or shroud. Also of note in this citation is the streamlined or “faired” body used to house the generator and associated mechanisms. This is a non-trivial design when it becomes necessary to insert the generator components within the high speed airflow of the turbine shroud or housing. Any non-essential drag in this area will directly subtract from the wind energy available for conversion. This cited application was assigned to the Grumman Aerospace Corporation which, one can assume at least, gave serious consideration to this invention. U.S. Pat. No. 4,140,433 entitled “Wind Turbine” to Eckel (1979) provides a further example of a venturi type, or shrouded turbine having components similar to those of modern turbojet aircraft engines. The wind turbine however is a passive device that must allow, rather than resist, the flow of wind through the turbine. In the present age it is a simple matter to determine, using computer airflow simulation, that the flow of wind, similar to the flow of electricity, will seek the path of least resistance. By definition, a venturi includes a restricted or narrowed passage that is presented as an obstacle to the flow of wind causing a dynamic pressure gradient extending well upstream of the wind turbine that uses a venturi of this type. This in turn causes a large proportion of the wind simply to divert around the turbine to rejoin well downstream or downwind of the device. This natural phenomena is appreciated in wind turbines using multiple concentrator wings having spacings between the concentrator wings that allow the wind to flow freely through and past the concentrator wings and, exactly as does an aircraft wing, develop a field of low static pressure that is used to create lift for aircraft, and suction or vacuum for wind turbines having multiple flow-through shrouds or concentrator wings. The use of flow-through shrouds or diffusers has been practiced for many years. A bi-wing or tri-wing aircraft uses similar principles to develop high lift from relatively low air speeds. U.S. Pat. No. 4,166,596 entitled “Airship Power Turbine” to Mouton, Jr., et al. (1979) deserves citation as such an example. In this example, the outer shroud or concentrator wing is referred to as the ‘vena contracta’, and includes a description of its function to induce a vacuum or low static pressure. The entire proposed structure is helium filled and designed to operate at higher altitudes in higher velocity winds. Whether or not practical, the aesthetic quality is uplifting. Aesthetics are important and in fact a distinct public criticism of wind farms in general is the visual marring of the natural landscape. The very rotational motion of large wind turbine rotor blades is perceived by many to distract and detract from the visual enjoyment of natural landscapes.
Further examples of wind turbines that use multiple shrouds or concentrator wings include:
U.S. Pat. No. 4,204,799 entitled “Horizontal Wind Powered Reaction Turbine Electrical Generator” to deGeus (1980);
U.S. Pat. No. 5,464,320 entitled “Superventuri Power Source” to Finney (1995);
An European Patent application No. EP1359320A1 entitled “Shrouded Fluid Flow Turbine” to Grassmann (published 2003).
In one embodiment the device of the present invention includes an aerobraking system that is reliable and potentially less costly than mechanical or aerodynamic braking systems of other rotor driven wind turbine systems. U.S. Pat. No. 4,565,929 entitled “Wind Powered System for Generating Electricity’ to Baskin, et al. (1986) describes a mechanical braking system that reacts to an increase in centrifugal force as the turbine rotor blades accelerate to trigger the deployment of aerodynamic drag producing devices affixed to the rotor blades that in turn slow the rotation of the blades. U.S. Pat. No. 4,715,782 entitled “Hydraulic Control Device for Wind Turbine” to Shimmel (1987) describes a similar operation but using hydraulics to deploy the aerobrake devices. U.S. Pat. No. 5,354,175 entitled “Wind Turbine Rotor Hub and Teeter Joint” to Coleman, et al. (1994) is another example of a mechanical braking scheme that allows the rotor blades to deflect backwards and in the direction of the wind flow under gusting or overwind conditions. This is similar to varying rotor blade pitch or angle of incidence but is perhaps faster to respond to gusting wind conditions. U.S. Pat. No. 6,265,785 entitled “Non-volatile Over Speed control System for Wind Turbines” to Cousineau, et al. (2001) is an example of a modern combined mechanical and aerodynamic system that uses a sophisticated mechanical, electrical and hydraulic system to prevent turbine overspeeding, and uses fail-safe systems to be initiated in the event of a power outage or failure of the hydraulics of the braking system. This latter example emphasizes the importance of reliable braking and back-up braking systems not only to prevent damage or destruction to the wind turbine and other wind turbines when within a wind farm, but as well to protect the public in the event of a catastrophic mechanical failure.
All of these devices add significant costs and many add considerable weight to the high speed rotor blades. Adding aerodynamic wing tip brakes and associated hardware to precisely toleranced and balanced rotor blades increases design, manufacturing and maintenance costs. As well, the added weight increases the demands made on the braking system itself and adds to the hazard should a component failure occur. To the point that it is effective, simple is surely better in the design of braking systems for wind turbines. The smaller turbines allowed by shrouded designs generally run at higher rpm's but can develop tremendous power depending on the size and number of concentrator wings used. Braking therefore is also an important consideration for shrouded wind turbines.
In recent years, significant research and experimentation has been undertaken at the University of Udine, Italy. An article prepared by a group of the University of Udine published in the journal of Renewable Energy (February, 2003) by Dr. H. Grassmann et al., is entitled “A Partially Static Turbine—first experimental results”. This article describes a prototype wind turbine having two shrouds or concentrator wings that allow a flow of air between the shrouds to develop an area of lower static pressure downwind of the turbine. An identical but unshrouded wind turbine is used for comparison. The article states that an increase of 100% of the power of the turbine was achieved in low wind velocities and 55% in high wind velocities. The lower percentage increase in power performance at high wind velocities (presumably 8 meters per second, or about 18 mph) is attributed, in the article, to turbulence generated by non-optimal impellor or rotor design. In the “Measurements” section the article states, “The simulation shows that consequently a large vortex behind the turbine is created. When one adds the shroud, this vortex strongly increases. As a result the shroud augments the power of the turbine by only 20% with these blades.” The article concludes at the end of this paragraph, “We strongly conclude at this point, that the quality of the propeller blades is very important for the performance of the shrouded turbine.” In the “Conclusion” section at the end of the article this is reinforced, “The quality of the propeller is decisive for the performance of such a system. A dedicated program of optimization is needed for the propeller.” The experimental results described in this article, for higher wind speed winds (although 18 mph is generally not considered high speed for wind turbines) show that as the vortex, or turbulence downstream of the wind turbine, increases with increasing wind speeds, the performance of the shrouded turbine markedly decreases. While it is given that the impellor blades produce a downstream turbulence it is not agreed that this turbulence is the cause of the “large vortex behind the turbine”. The research of the applicant has demonstrated that the generation of the turbulent vortex is more fundamental and would occur even if the impellor blades were to present at all. The powerful stream of air that is drawn by suction through the smallest diameter shrouds forces directly downstream and interferes with the flow of wind over and between the concentrator wings that are attempting to flow the wind outwards, away from the central axis. These are contrary forces, and in higher wind conditions, as the article indicates, the stronger force wins with the resultant formation of a large turbulent vortex, the aerodynamic stalling of the concentrator wings, and the loss of power. The phenomenon is analogous to a blow torch that blows itself out when too much gas pressure is applied. It is therefore a significant object of the present invention to introduce a flow regulator element installed in the downstream flow of air that is drawn through the turbine or smallest diameter shrouds so as to stabilize either the force of air flowing out of the turbine or smallest diameter shrouds or flowing through the impellor blades of the device. In so doing, it is unnecessary to optimize the impellor blades, certainly a futile attempt at least for higher speed winds. An additional use of the flow regulator element is as part of an aerobraking system that serves to respond quickly to wind gusts and control or restrict the flow of wind through the impellor and thereby protect the turbine from overspeeding in gusting or overly high wind conditions.